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.Dd $Mdocdate: January 15 2015 $
.Dt INFNAN 3 vax
.Os
.Sh NAME
.Nm infnan
.Nd signals invalid floating\-point operations on a VAX (temporary)
.Sh SYNOPSIS
.In math.h
.Ft double
.Fn infnan "int iarg"
.Sh DESCRIPTION
At some time in the future, some of the useful properties of
the Infinities and \*(Nas in the IEEE standard 754
for Binary Floating\-Point Arithmetic will be simulated in UNIX
on the DEC VAX by using its Reserved Operands.
Meanwhile, the Invalid, Overflow and Divide\-by\-Zero exceptions of the
IEEE standard are being approximated on a VAX by calls to a procedure
.Fn infnan
in appropriate places in
.Em libm .
When better exception\-handling is implemented in UNIX, only
.Fn infnan
among the codes in
.Em libm
will have to be changed.
And users of
.Em libm
can design their own
.Fn infnan
now to
insulate themselves from future changes.
.Pp
Whenever an elementary function code in
.Em libm
has to
simulate one of the aforementioned IEEE exceptions, it calls
.Fn infnan iarg
with an appropriate value of
.Fa iarg .
Then a
reserved operand fault stops computation.
But
.Fn infnan
could
be replaced by a function with the same name that returns
some plausible value, assigns an apt value to the global
variable
.Va errno ,
and allows computation to resume.
Alternatively, the Reserved Operand Fault Handler could be
changed to respond by returning that plausible value, etc.,
instead of aborting.
.Pp
In the table below, the first two columns show various exceptions
signaled by the IEEE standard, and the default result it prescribes.
The third column shows what value is given to
.Fa iarg
by functions in
.Em libm
when they
invoke
.Fn infnan iarg
under analogous circumstances on a VAX.
Currently
.Fn infnan
stops computation under all those
circumstances.
The last two columns offer an alternative;
they suggest a setting for
.Va errno
and a value for a
revised
.Fn infnan
to return.
And a C program to implement that suggestion follows.
.Bl -column "IEEE Signal" "IEEE Default" "+-ERANGE" "ERANGE/EDOM" "infnanXX"
.It Sy "IEEE Signal" Ta Sy "IEEE Default" Ta Fa iarg Ta Va errno Ta Fn infnan
.It Invalid Ta \*(Na Ta Dv EDOM Ta Dv EDOM Ta 0
.It Overflow Ta \(+-\*(If Ta Dv ERANGE Ta Dv ERANGE Ta Dv HUGE
.It Div\-by\-0 Ta \(+-\*(If Ta Dv +-ERANGE Ta Dv ERANGE/EDOM Ta Dv +-HUGE
.El
.Pp
.Dl ( Ns Dv HUGE No = 1.7e38 ... nearly  2.0**127)
.Pp
ALTERNATIVE
.Fn infnan :
.Bd -literal -offset indent
#include <math.h>
#include <errno.h>

double
infnan(int iarg)
{
	switch (iarg) {
	case \0ERANGE:
		errno = ERANGE;
		return (HUGE);
	case \-ERANGE:
		errno = EDOM;
		return (\-HUGE);
	default:
		errno = EDOM;
		return (0);
	}
}
.Ed
.Sh SEE ALSO
.Xr intro 2 ,
.Xr signal 3
.Sh HISTORY
The
.Fn infnan
function appeared in
.Bx 4.3 .
